Vibrating structure gyroscopes provide one example of a micro electro mechanical systems (MEMS) sensor device formed from a semiconductor e.g. silicon substrate. They can be batch fabricated from silicon wafers using conventional micromachining techniques. There is considerable interest in utilising MEMS gyroscopes in a range of guidance, navigation and platform stabilisation applications due to their low cost, small size and inherently robust nature. However, the limited performance capability of MEMS devices limits their wide-scale deployment in these areas. One performance limiting feature is that of rate bias stability e.g. resulting from variations in temperature during operation of the device.
Some examples of vibrating structure gyroscopes may be found in GB 2322196, U.S. Pat. Nos. 5,932,804 and 6,282,958. Each flexible support 4a to 4h includes a pair of compliant legs 8a, 8b that are attached at one end to the external periphery of the annular resonator 2 and at the other end to the internal periphery of a support frame 10 defined by the substrate 6. The flexible supports 4a to 4h allow the annular resonator 2 to vibrate in response to electromagnetic drive transducers (not shown) constituted by metal track sections on two of the supports. Primary and secondary pairs of inductive pick-off transducers (not shown) are constituted by metal track sections on other of the supports. Such a vibrating structure gyroscope may have the annular resonator made of silicon and be particularly suited for fabrication using micromachining techniques.
In a typical vibrating structure gyroscope, the annular resonator is typically excited into a cos 2θ resonance mode. For a perfectly symmetrical resonator, this mode actually exists as a degenerate pair of primary and secondary vibration modes at a mutual angle of 45°. The primary mode is excited as the carrier mode. When the annular resonator is rotated about an axis normal to its plane, the Coriolis effect causes a secondary vibration in an orthogonal direction that couples energy into the secondary response mode. The amplitude of motion of the secondary response mode is proportional to the applied angular velocity.
In such devices, a quadrature bias may arise due to an imperfect matching of the primary and secondary frequencies in the cos 2θ resonance mode, which are ideally set to be equal. The magnitude of the quadrature bias is proportional to ΔF.sin 4α, where ΔF is the mode frequency split and α is the mode angular alignment with respect to the primary drive axis. Conventionally the quadrature bias is minimised during production using a laser trimming process which sets ΔF approximately equal to 0 Hz at room temperature. However, the Applicant has recognised that stress and strain induced due to the thermal expansion coefficient mismatches between various materials used in construction of a gyroscope can cause the ΔF value, and hence the quadrature bias, to change over the operating temperature range of the device.
FIG. 2 shows typical data for the quadrature bias variation from an initial room temperature value for a range of sensors of the type described in GB 2322196. The average variation is around 150° per second over the −40 to +85° C. measurement range. This signal is in phase quadrature to the desired rate bias signal and is largely rejected by the control electronics. In practice, however, component tolerances in the electronics will introduce errors limiting the phase accuracy, thus allowing some of the quadrature bias to break through into the rate bias channel. Both the quadrature bias and phase error vary with temperature, giving rise to a variation in the rate bias. The instability of the rate bias in such conventional vibrating structure gyroscopes results in a performance that is unsatisfactory for many sensitive applications.
FIG. 3 shows a schematic cross-section of the sensor head structure of the vibrating structure gyroscope described in GB 2322196. An annular resonator 1 is seen to be supported spaced from a silicon substrate 20, while the silicon substrate layer 20 is mounted to a Pyrex glass pedestal layer 22 and a Pyrex glass spacer layer 24 to form a MEMS structure. The MEMS chip is attached by a die bond layer 26 e.g. of a silicone elastomer adhesive, to a rigid can package base 28. According to GB 2322196, the can package housing 28 is made from Kovar material i.e. a nickel-iron alloy. The different coefficients of thermal expansion for the various materials used in the construction of the MEMS device will induce stresses and strains that will vary with the ambient temperature of the device. For example, the coefficient of thermal expansion for silicon is 3.2 ppm per ° C. and the coefficient of thermal expansion for Pyrex is 3.25 ppm per ° C., whereas the coefficient of thermal expansion for nickel-iron alloy (e.g. NILO 45) is 7 ppm per ° C. Because the MEMS chip is square, it tends to resist the stress-induced deformation to a larger extent around the corner areas, which are more rigid due to their greater width. This can result in an asymmetric stress and strain distribution which has peaks and troughs which are angularly aligned with the corners and faces of the square chip. This variable stress and strain can be coupled into the annular resonator 1 through the legs 8a, 8b of supports 4a-4h. This effectively imparts a cos 4θ perturbation into the resonator 1 which can induce a frequency split ΔF between the cos 2θ modes which will vary with ambient temperature.
A significant source of asymmetric stress and strain arises due to the large thermal expansion mismatch between the package housing 28 and the MEMS device. The die bond layer 26 is intended to decouple the MEMS device from the stress and strain of the package housing 28. However, such a silicone elastomer adhesive material typically has a comparatively low Young's Modulus (0.1×109 Pa) compared to that for the silicon layers (190×109 Pa) and Pyrex layers (62.7×109 Pa), but has a very high thermal expansion coefficient (500 ppm per ° C.) as compared to 3.2 ppm per ° C. for silicon and 3.25 ppm per ° C. for Pyrex. The net result is that significant stress and strain can be imparted to the MEMS structure from the elastomer die bond layer 26 and this may introduce a significant asymmetry with a cos 4θ periodicity. The square chip structure acts to focus this asymmetry so that thermally-induced stress and strain splits the frequencies of the cos 2θ mode pair.
The present disclosure seeks to address at least some of the issues outlined above.